Turbochargers that are used on both diesel and gasoline engines to increase power output, reduce fuel consumption and reduce emissions have reached an advanced stage of development. The higher mileage and emission standards set by the Obama administration, which begin to take effect in 2012 and are to be achieved by 2016, will necessitate changes to the American car and truck fleet. Passenger cars will be required to get 39 miles per gallon, and light trucks to get 30 miles per gallon. Some U.S. car manufacturers are already taking advantage of turbochargers to help achieve these goals. A news release announced in May of 2009, that Ford Motor Company had started producing its EcoBoost V-6 turbocharged engine, said to boost fuel economy by 20 percent and reduce CO2 emissions by 15 percent. It is likely that other engine manufacturers will follow Ford in the use of turbochargers to take advantage of their ability to increase power from smaller engines while, at the same time, improve fuel mileage and lower emissions. Anything that improves performance and lowers the cost of small turbochargers will be a help in this effort.
The compressor component of a turbocharger comprises a compressor wheel, driven by the turbine, a diffuser section peripherally outboard of the compressor wheel, and a surrounding casing that collects the air from the diffuser and delivers it to the air intake system of the engine.
The compressor wheel imparts a static pressure increase to the air and accelerates it to a high velocity at its outlet. This static pressure rise is customarily about half of the total pressure rise that occurs in the compressor. The remaining half of the total pressure rise occurs in the diffuser that converts the high air velocity, leaving the wheel, to pressure by decelerating the air as it passes through the diffuser section.
A typical turbocharger compressor that compresses 80° F. ambient intake air to three times atmospheric pressure will have an air discharge temperature of about 350° F., if its efficiency is 73%. Half of this temperature rise occurs in the compressor wheel so that the air temperature at the wheel outlet will be approximately 175° F. This is low enough for it to be used as a cooling media.
Most commercial turbochargers now use floating sleeve bearings that are capable of suppressing shaft instability and have achieved satisfactory durability on a variety of internal combustion engines. The floating sleeve bearing systems used in commercial turbochargers include a stationary thrust bearing to carry the axial loads generated by the rotor assembly. The friction loss associated with the thrust bearing, plus the friction losses in the inner and outer oil films of the floating sleeve bearings, results in a substantial total friction loss for the complete bearing system. This friction loss has been the motivation for the development of ball bearing systems for small turbochargers that have much lower friction losses and allow the turbocharger rotating assemblies to accelerate appreciably faster than those using the sleeve bearing systems. See U.S. Pat. No. 7,677,041 B2 for the disclosure of a successful ball bearing system.
The ball bearing systems require a means of carrying away the heat generated in the bearings when operating at very high speeds in small turbochargers. Normally, this is accomplished by using engine lubricating oil ducted through and around the bearings, and then returning the oil through piping to the engine crankcase. In some gasoline engine applications, where exhaust gas temperatures are high, the lube oil cooling is augmented by a cooling jacket in the bearing housing through which engine coolant is circulated to accomplish sufficient cooling of the bearings and the internal structural parts of the turbocharger. In applications where such cooling of the bearings is required, the location and installation of the turbocharger is complicated and made more expensive by the requirement for piping for the engine lubricant and/or engine coolant between the turbocharger and the internal combustion engine.
Engines that are required to produce high power at low engine speeds, diesel truck engines for example, or passenger car engines that need to accelerate quickly, require turbochargers that are capable of supplying as high an air charge pressure as possible over the low engine speed range, up to the torque peak speed of the engine. To accomplish this, small-size turbines that have turbine casings with small throat areas, are used to force the turbocharger to rotate at as high a speed as possible over the low speed range of the engine. In order to prevent these turbines from operating the turbocharger above its rated speeds over the high speed range of the engine, exhaust gas bypass valves, knows as waste gates, are employed to bypass exhaust gas around the turbine wheel to limit the maximum speed of the turbocharger.
A predetermined maximum air charge pressure is used to actuate the waste gate that is usually built into the turbine casing and, by bypassing exhaust gas around the turbine wheel, the turbocharger speed can be held constant over the high speed range of the engine, namely, above the torque peak speed, where there is excess energy in the engine exhaust gas. This system requires a mechanism actuated by compressor discharge pressure to open the waste gate valve that is, as previously stated, usually built into the turbine casing. The waste gate valve and its operating mechanism represent a significant cost addition to the basic turbocharger.
Notwithstanding years of turbocharger design, development and production, the use of turbochargers remains complicated by their need for cooling of the internal parts of the turbocharger, including the bearings that carry the rotating assembly of the turbocharger and are heated, not only by the friction losses of the bearings themselves, but also by the heat conducted through the rotating shaft from the turbine wheel, which is exposed to the hot exhaust gases of the internal combustion engine. The need for turbocharger cooling has required piping for delivery of the lubricant and/or the coolant from the internal combustion engine to the turbocharger at its location within the engine enclosure.